1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like.
The channels in a wireless communication system are defined by one or more transmission protocols. For example, in wireless communication systems that operate according to the Frequency Division Multiple Access (FDMA) protocol, the transmission bandwidth allocated to the air interface is divided into a number of frequencies and these frequencies are allocated to the various channels. For another example, Code Division Multiple Access (CDMA) protocols implement coding sequences that may be used to modulate transmitted information so that multiple users may transmit on the same frequency or set of frequencies. Other transmission protocols include Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-FDMA (SC-FDMA). In an OFDMA system, the available bandwidth may be divided into a plurality of orthogonal subcarrier frequencies (commonly referred to as subcarriers), which may be allocated to one or more subchannels so that multiple users may transmit data concurrently using separate groups of subchannels. In SC-FDMA, the available bandwidth is also divided into a plurality of orthogonal subcarriers similar to OFDMA, but discrete Fourier transform (DFT) pre-coding is used to provide low Peak-to-Average-Power Ratio (PAPR) compared with OFDMA transmission.
Conventional wireless communication systems typically implement techniques for ensuring timing synchronization between the base stations and the mobile units. However, these wireless communication systems typically do not implement techniques for frequency synchronization between the base stations and the mobile units. Consequently, a frequency offset may be formed between the expected frequency of each sub-carrier and the actual frequency that is transmitted and/or received by a base station or mobile unit. For example, wireless communication protocols such as the Evolved UMTS Terrestrial Radio Access (E-UTRA) system require network operation at user equipment (UE) speeds of up to 350 kmph or even at higher speed of 500 kmph. The maximum frequency offset foffset seen at an eNode B receiver may be expressed as:foffset=ΔfBS+ΔfUE+2×fDoppler—max  (Eq. 1)where ΔfBS, ΔfUE, and fDoppler—max denote the base station frequency drift, the UE frequency error, and the maximum Doppler frequency, respectively. In the UMTS W-CDMA system, the frequency error at the base station and the UE is required to be less that 0.05 ppm and 0.1 ppm of the carrier frequency, respectively. For a carrier frequency of 2.1 GHz, the maximum frequency offset is therefore 781 Hz for a UE moving at the velocity 120 kmph, and can be as large as 2260 Hz, when the velocity is 500 kmph.
FIG. 1 shows the frequency offset of one mobile unit as a function of the velocity of the mobile unit. The vertical axis indicates the frequency offset in Hertz and the horizontal axis indicates the velocity in kilometers per hour. The frequency offset at 0 kph corresponds to the base station frequency drift and the user equipment frequency error. The frequency offset increases approximately linearly with increasing velocity as the Doppler shift of the moving mobile unit increases. In FIG. 1, the frequency offset increases from approximately 350 Hertz to approximately 2260 Hertz.
FIG. 2 shows the normalized frequency offset of one mobile unit as a function of the velocity of the mobile unit. In the FIG. 2, the frequency offset is normalized to the sub carrier spacing. For example, the normalized frequency offset (denoted as ε) is obtained as
                    ɛ        =                              f            offset                                Δ            ⁢                                                  ⁢                          f                              sub                ⁢                                  -                                ⁢                carrier                                                                        (                  Eq          .                                          ⁢          2                )            The vertical axis indicates the normalized frequency offset as a percentage of the subcarrier spacing and the horizontal axis indicates the velocity in kilometers per hour. The normalized frequency offset at 0 kmph corresponds to the base station frequency drift and the user equipment frequency error. In FIG. 2, the normalized frequency offset at 0 kmph is approximately 0.03 of the subcarrier spacing. However, the Doppler shift causes the normalized frequency offset to increase to approximately 23% of the subcarrier spacing at velocities of 500 kph. Furthermore, when there are two UEs travelling in opposite directions, the amount of residual frequency offset between the two UEs can be twice that of the frequency offset between a single UE and the eNodeB.
FIGS. 3 and 4 show the impact of frequency offset on the modulation symbol in a SC-FDMA system. The vertical axes in these figures indicate the imaginary part of the modulation symbol and the horizontal axes indicate the real part of the modulation symbol. FIG. 3 shows a received signal constellation in the presence of frequency offset for QPSK modulation and a normalized frequency offset of 0.1. FIG. 4 shows a received signal constellation in the presence of frequency offset for 16-QAM modulation and a normalized frequency offset of 0.1. As in any single-carrier system, frequency offset introduces rotation of the signal constellation. The amount of rotation depends on the sampling rate and the symbol duration. Conventional wireless communication systems implement a frequency offset estimation and compensation algorithm on the receiver-side. The frequency offset estimation and compensation algorithms can rotate the signal approximately back to the original constellation. However, in a multi-user FDM system, frequency offsets also exacerbate inter-carrier interference (ICI) and these effects cannot be corrected by simply applying a frequency offset estimation and compensation algorithm, at least in part because of the difficulty in separating contributions from symbols transmitted using the different carriers with different frequency offset. In some cases, the ICI can be significant and limit the performance of multi-user SC-FDMA system. The signal degradation caused by ICI may be particularly acute when users with different received SNR requirements are scheduled in adjacent subcarriers.
FIG. 5 shows the degradation in received signal power due to frequency offset. The vertical axis indicates the received signal power degradation in decibels and the horizontal axis indicates the normalized frequency offset. In the illustrated embodiment, ICI is assumed to be produced by a single interfering user. The symbol SNR of the user of interest and the symbol SNR of the interfering user are assumed to be equal. Increasing the normalized frequency offset causes the received signal-to-interference-plus-noise ratio (SINR) to be increasingly degraded. FIG. 5 shows that the SINR can degrade up to 4 dB when the frequency offset is large.
FIG. 6 shows the degradation in the signal-to-interference-plus-noise ratio (SINR) caused by ICI between users having a normalized frequency offset. The vertical axis indicates the approximate SINR and the horizontal axis indicates the normalized frequency offset. FIG. 6 shows that even in the presence of small residual frequency offset, the amount of SINR degradation can be significant. The degradation of the SINR is larger when the SNR is large. Although the symbol constellation of the user of interest may be rotated to approximately the original constellation, conventional frequency offset estimation and compensation algorithms are not typically able to compensate for frequency offset of the interfering user. Consequently, ICI from the frequency offset signal from the interfering user may result in significant degradation in the SINR for the user of interest. This can be a limiting factor in achieving a high target data rate on the uplink in high date rate systems such as the Long Term Evolution (LTE) system.